The present invention relates to a microwave ceramic filter having a ceramic body with at least two resonator bores and to a method for adjusting the filter characteristic of this filter.
Microwave ceramic filters are known from U.S. Pat. Nos. 5,146,193 and 5,369,011. These filters have a ceramic body with at least two resonator bores, which are coupled to one another via the ceramic body and which form ceramic resonators. The ceramic body is metallized all-around including the inside surface of the resonator bores, whereby input/output connections are formed on a surface by recesses or openings in the metallization.
Given a filter of the described species, it is further known from the cited printed patent specifications that the filter characteristic has at least one stop frequency, which can be influenced by the size of the recess surrounding the input/output connections.
In practical operation, it is frequently the case that users of HF filters, e.g. in the mobile radio telephone service, construct different heterogeneous receivers having different intermediate frequencies. This frequently forces the producers of such filters to short-term adapt the mirror suppression of the filter to the requirements of the customer or, in the most unfavorable case, to construct it for selective filtersxe2x80x94predominantly multipolar filtersxe2x80x94which are suitable for different attenuation requirements and which, however, are correspondingly complicated to produce and therefore expensive. Besides, microwave ceramic filters have the disadvantage that higher selectivity also conditions higher insertion attenuation.
An object of the present invention is to create a microwave ceramic filter of the relevant type, wherein the notch frequency can be shifted to the higher frequency edge of the filter characteristic, so that a better selection is obtained at the selected image frequency without the insertion attenuation suffering.
This object is achieved by a microwave ceramic filter having at least two ceramic resonators, which are coupled to one another and which are formed by resonator bores provided in a ceramic body and by an all-around metallization of the ceramic body and of the inside surfaces of the resonator bores, the metallization being partially left open for forming input/output connections and a ground or grounding connection, the improvement being a non-metallized strip extending from a first edge of the ceramic body to a second edge of the ceramic body, which is opposite the first edge, between the input/output connections and the grounding connection, and by a method for adjusting the filter characteristics of the microwave ceramic filter by adjusting a width of the non-metallized strip extending between the input/output connections and the ground connection from the first edge to the second edge. An advantageous further development of the invention includes an additional or further ground connection being provided between the input/output connections.